In electrophotography, a photoreceptor containing a photoconductive insulaing layer on a conductive layer is imaged by first uniformly, electrostatically charging its surface. The member is then exposed to a pattern of activiting electromagnetic radiation, such as light. The radiation selectively dissipates the charge in the illuminated area of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated area. This electrostatic latent image may then be developed to form a visible image by depositing finely divided toner particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred to a support such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
A photoreceptor may exist in a number of forms. For example, the photoreceptor may be a homogeneous layer of a single material or may be a composite of more than one distinct layer. An example of a multilayered photoreceptor may comprise a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer and a charge transport layer. U.S. Pat. No. 4,265,990 discloses a photoreceptor having at least two electrically operative layers, including a charge generating layer and a charge transport layer.
In multilayered photoreceptors, materials used for each layer preferably have desirable mechanical properties while also providing electrical properties necessary for the function of the device. If the material of one layer of the photoreceptor or a process used to prepare it is changed in an attempt to improve a particular property, e.g., an electrical property, the change may have an adverse effect on other properties, e.g., mechanical properties, such as delamination of one or more layers. Similarly, if the materials comprising the layers or the processes used to prepare and apply the layers are altered, the photoreceptor sensitivity, response and useful life may be affected.
Suitable and economical coating methods used for applying layers in multi-layer photoreceptors include dip coating, roll coating, Meyer bar coating, bead coating, curtain flow coating and vacuum deposition. These exemplary methods are known in the art. Solution coating using any of the above methods is a preferred approach.
Known vacuum deposition processes are useful in applying charge generating material to an underlying substrate to form a charge generating layer. Conventional photoreceptor devices having vacuum deposited charge generating layers generally have greater photosensitivity than devices having charge generating layers prepared and applied using other known application processes, notably, devices having a charge generating layer comprised of a matrix of resin binder and photogenerating material.
U.S. Pat. Nos. 4,587,189 to Hor et al., 4,882,254 to Loutfy et al. and 4,921,773 to Melnyk at al., the disclosures of which are entirely incorporated herein by reference, disclose charge generating layers, particularly, charge generating layers which have been vacuum deposited.
Improved photoreceptors having an extended electrical life may require a thin charge generating layer, preferably less than 1 .mu.m. In order to assure adequate optical absorption for the charge generating layer in these thin layers, the photogenerating pigment may be dispersed in a polymeric host matrix, but may need to be present at higher concentration than required for photoreceptors having thicker (e.g., 2 .mu.m to 3 .mu.m) photogenerating layers.
U.S. Pat. No. 4,082,551 to Steklenski et al. discloses a photoconductive insulating composition employed in multi-layer elements. The composition may be composed of a wide variety of organic, including organometallic materials in admixture with an electrically insulating film-forming binder material. The disclosed photoconductive compositions are prepared by blending a dispersion or solution of the photoconductive material together with a binder and coating or otherwise forming a layer of such photoconductive composition on an underlying layer. No purification or pretreatment of the photoconductive material is disclosed.
U.S. Pat. No. 4,571,371 to Yashiki discloses an electrophotographic photosensitive member having a charge generating layer and a charge transport layer. A dispersion of charge generating material dissolved in solvent was applied to a cured polyamide resin layer by soaking, and was dried at 100.degree. C. for 10 minutes to form a charge generating layer. Disclosed, exemplary photoconductive materials include phthalocyanine pigment powders and the like, or organic photoconductive materials. No purification or pretreatment of the photoconductive materials is disclosed.
Photogenerating pigments used in charge generating compositions for charge generating layers can be purified to improve photosensitivity of photoreceptor devices. One such purification process involves sublimation of a photogenerating pigment and subsequent condensation of the sublimed pigment. In conventional sublimation apparatus used to purify photoconducting pigments, because a conventional collector is heated both directly by a crucible and by release of heat due to condensation of the purified pigment, the conventional collector undergoes a temperature increase during condensation of the purified pigment.
DE 40 31 898 A1 to Nishiwaki et al. discloses a process for producing and recovering ultrafine particles, such as organic photoconductive particles for electrophotographic photoreceptors, by vapor deposition. In the disclosed process, a particle carrier is moved within a gas phase in a section of a chamber and the ultrafine particles are evaporated by heating the material, which can be vaporized to be laid on a moving carrier. The laid particles are collected while further particles are deposited on another part of the carrier. The deposited particles are collected by a scraper blade, a brush, by suction or by stripping. The material is evaporated at a temperature of more than 80.degree. C. and the particle-charged content is cooled at less than 10.degree. C. The chamber section is evacuated to 10.sup.-2 to 10.sup.2 Torr. The moving carrier is subject to undesirable high temperature effects of conventional sublimation apparatus and the disclosed process is carried out at pressure greater than or equal to 10.sup.-2 Torr.
U.S. Pat. No. 4,220,697 to Wiedemann discloses an electrophotographic recording material comprising a photoconductive layer composed of at least one layer comprising charge carrier-producing and charge transporting compounds. A homogeneous, tightly packed dyestuff layer is achieved by vapor-deposition of the dyestuff on the support under reduced pressure. A vacuum layer between 10.sup.-3 and 10.sup.-5 Torr and heating temperature of between 250.degree. and 400.degree. C. results in vapor deposition without decomposition. The temperature of the support is below 50.degree. C. Charge generating layer dispersions using the vapor-deposited dyestuffs are not disclosed.
U.S. Pat. No. 4,578,334 to Borsenberger et al. discloses multi-active photoconductive insulating elements comprised of a charge generation layer and a charge transport layer. The insulating elements are prepared by 1) depositing, on an electrically-conductive support, a substantially amorphous layer of N,N'-bis(2-phenethyl)perylene-3,4,9,10-bis(dicarboximide), hereinafter "PPC"; 2) overcoating the substantially amorphous layer with a liquid composition comprising an organic solvent; and 3) effecting removal of the organic solvent from the element. The function of the liquid composition is to (A) form a charge transport layer and (B) to penetrate into the amorphous layer and convert the PPC to crystalline form. Vacuum deposition of the PPC is carried out at a pressure from about 10.sup.-4 to 10.sup.-6 Torr and at a crucible temperature ranging from about 250.degree. C. to 450.degree. C., while maintaining a substrate temperature at or below room temperature. The vacuum-deposited PPC is not crystalline and does not result in a resin dispersed charge generating layer.
U.S. Pat. No. 4,431,722 to Takei et al. discloses a photosensitive element for electrophotography comprised of a layered structure having a polycyclic quinone pigment dispersed in an organic resin binder as a charge generating layer. A commercially available polycyclic quinone is vacuum evaporated for 5 minutes at a temperature of 350.degree. C. and deposited on a substrate disposed 15 centimeters above the evaporation source. The thus prepared pigment is dispersed in a liquid and may be incorporated with a binder resin to improve the mechanical strength and adhesion of the resulting charge generating layer. Vacuum evaporation of the pigment at reduced pressures less than 10.sup.-3 Torr and collection of the sublimed pigment on a cooled substrate are not disclosed.
Conventional photoreceptors, having at least a charge generating layer and charge transport layer and made according to a conventional process, suffer numerous disadvantages. For example, some photoreceptors suffer from poor charge acceptance. Notably, devices manufactured using conventional vacuum deposition processes have vacuum deposited charge generating layers without a resin component, thus resulting in less durable photoreceptors.